Dip process for forming transparent surface conversion coatings on zinc, and compositions for dip solutions



United States Patent l DIP PROCESS FOR FORMING TRANSPARENT SUR- FACECONVERSION COATINGS ON ZINC, AND COMPOSITIONS FOR DIP SOLUTIONS Jesse E.Stareck, Royal Oak, Mich, assignor, by mesne assignments, to Metal &Thermit Corporation, New York, N. Y., a corporation of New Jersey N0Drawing. Application April 20, 1954 Serial No. 424,786

8 Claims. (Cl. 148'- 6.21)

This invention relates to a dip process for forming a surface conversioncoating on zinc and high zinc alloys, to a dip solution for use in theprocess, and to a composition for making the dip solution. Moreparticularly, the invention provides a one-step dip process for forminga bright, colorless, transparent, protective surface conversion coatingon zinc and is further characterized as follows:

(1) The solution does not etch or dull zinc.

(2) Brightening of the ztnc is accomplished in a short time, from a fewseconds up to two minutes.

(3) The basic metal under zinc coatings is not exposed by stripping ofZinc in the time required for brightening.

(4) The conversion coating will stand hot water treatment withoutdisintegrating or cracking.

(5) The coating which is formed is uniform from a practical standpoint.

(6) The article with the surface conversion coating formed thereon hasgood salt spray resistance.

(7) Fingermarks which occur on bright zinc when handled are notobservable when bright zinc covered with the transparent surfaceconversion coating is handled.

(8) The surface coating is transparent, exhibiting the underlying zincas bright metallic Zinc.

(9) the dip solution has a reasonably long useful life, being butteredtoprovide a retarded rise of the pH.

(10) Surface conversion coatings having the foregoing characteristicsare formed in both still and agitated immersion.

The aqueous dip solution has the following composition, expressed interms of the essential substances, for which a preferred concentrationand an over-all or range of concentrations are given along withconditions of pH and temperature of the solution:

The designation hexavalent chromium, Cr is to be understood asrepresenting chromium in a form combined with at least one otherelement, the quantity however being based on the designation Cr;hexavalent chromium, Cr may alternatively be expressed in terms of CrOthe preferred concentration of which is 192.3 g./l: and the over-allconcentration of which is 48.1 to 288.5 g./l. The designation up to 80for acetate represents that .some acetate is always present, usually atleast 1 gi/l. of acetate. Thesource of eachof the foregoing substancesis variable provided that the resulting solution has a pH between -0.3and +1.0. Examples of source materials are chromic acid, sodiumbichromate and its dihydrate, sodium trichrornate, sodium tetraehromateand its tetrahydrate, potassium bichromate,

potassium trichromate, potassium tetrachromate, ammonium bichromate,ammonium trichromate, ammonium tetrachromate, zinc bichromatetrihydrate, zinc trichromate trihydrate, sodium 'chromate and thetetra-, hexa-, and decahydrates of the same, potassium chromate, lithiumchromate, zinc chromate, ammonium chromate, ammonium lithium chromate,ammonium potassium ichromate, ammonium sodium chromate, potassium sodium chromate, tripotassium rnonosodiurn bichromat'e, potassium calciumchromate, potassium magnesium chromate and its diand hexahydrates,potassium zinc chm-mate dihydrate, magnesium chromate, rubidiumchromate, cesium chroina'te, sulfuric acid, sodium bisulfate, potassiumbisulfate, lithium bisulfate, rubidium bisulfate, cesium bisulfate,sodium sulfate, potassium sulfate, lithium sulfate, zinc sulfate,magnesium sulfate, rubidium sulfate, cesium sulfate, nitric acid,potassium hydrogen nitrate, rubidium hydrogen nitrate, cesium hydrogennitrate, sodium nitrate, potassium nitrate, lithium nitrate, zincnitrate, calcium nitrate, ammonium nitrate, magnesium nitrate, rubidiumnitrate, cesium nitrate, acetic acid, sodium hydrogen acetate, potassiumhydrogen acetate, sodium acetate, potassium acetate, lithium acetate,zinc acetate, calcium acetate, ammonium acetate, magnesium acetate,rubidium acetate, and cesium acetate;

The amount of a source material will, of course, be equivalent to theamount of substance required. All four substances may be supplied in theform of acid source materials as in Example 3 below, or in the form ofmixed acid and salt source materials as in Examples 1 and 2, or in theform of acid salt source materials: provided always that the pH of theresulting solution is between --0.3 and +1.0.

Specifice examples of the qualitative and quantitative composition ofsome dip solutions are as follows:

Example 1 G./l. Chromic acid, C r O 200 Sodium sulfate, Na SO 6 Sodiumbisulfate, NaHSO 24 Sodium nitrate, NaNO Sodium acetate, NaC H O 55ExampleZ Chromic acid, CrO 150 Sodium bichrornate dihydrate, Na Cr O -2HO Sodium bisulfate, NHHSO4 30 Sodium nitrate, NaNO 15 Sodium hydrogenacetate, NaH(C I-I O Example 3 Chromic acid, CrO Sulfuric acid, H SO 8Nitric acid, HNO 3.5 Acetic acid, HC H O 13 Generally, the nitrateconcentration varies inversely with the hexavalent chromiumconcentration, and the sulfate and acetate concentration vary directlywith the hexavalent chromium concentration.

The use-of lowerternperatures within the above ranges is advantageous.

The time of immersion in the dip solution is from about 5 seconds to 2minutes, and is usually from 10 to 20 seconds. The time of immersion islonger with higher pH, lower temperature, and lower concentration; andthe time is shorter for lower pH, higher temperature, and higherconcentration.

The action of the solution on the zinc is to form a surface conversioncoating of definite thickness, i. e. on the order of about two to fivemillionths of an inch. With prolonged immersion, the action of thesolution on the zinc continues but the coating does not get appreciablythicker.

With an ordinary solution of a chromate or chromic acid, and a sulphate,the zinc acts to reduce the chromate or chromic acid, and a browncoating is formed. Nothing 5 desirable in the way of brightening thezinc is obtained, and the tendency is toward the formation of roughcoatings. In a solution made up according to the invention, the nitratecounteracts the formation of the brown color of the coating. Being asmaller ion than the chromate ion, the nitrate ion more readilypenetrates to the zinc, and the zinc reduces it rather than thechromate. With increasing amounts of nitrate up to the preferred amount,the surface conversion coatings which are formed become increasinglylighter in color, until a transparent coati '2 without any apparentbrown coloration is obtained. Tn, nitrate also acts to widen the pHrange at which the dip solution has a brightening action on the zinc.

A chief function of the acetate radical is to limit the thickness of thesurface conversion coating; if the coating #0 is too thick, it willeither crack or form a powder when treated with hot water. The acetatealso enables the pH range to be wider; one can go higher in the pH rangewhen acetate is present, and this in turn prolongs the useful life ofthe dip solution, which becomes less and less acid with use. The acetateappears to buifer the hydrogen ion concentration. Chrornic acid in thesolution also acts to buffer the hydrogen ion concentration. Anotherfunction of the acetate radical appears to be to increase the solubilityof trivalent chromium com- 00 pounds which may be found in the dipsolution and thereby keeps them from precipitating in the coating. Theacetate has a further effect of increasing the smoothness of thecoating.

A composition useful for making up or maintaining dip solutions may bebroadly defined as consisting essentially of the following substances inthe following amounts:

Parts by weight Cr 25 to 150 S0 5 to 60 N03 2 t0 CzHgOg to of whichvaries from 48.1 to 288.5 parts by weight. The

composition is essentially characterized by the fact that it produces asolution having a pH between 0.3 and 1.0 when the source materialscomprising the composition are dissolved in water to provideconcentrations of the dissolved substances which are within theconcentration ranges defined above in connection with the dip solutions.Illustrative source materials are those already noted, some of which arenormally solid and some of which are nortnally liquid, and as will beapparent, the resulting composition may be either in the solid or theliquid state depending on the source materials of which it is composed.Solid and liquid compositions each possess certain advantages over theother. Thus, solid compositions are easier to package and transport, andsome users prefer them to handle and to store; by contrast, liquidcompositions are put up in glass carboys which are less convenient toship and are subject to breakage, in the latter event a highly corrosiveliquid being set loose, and also liquid compositions involve higherfreight expenses because of the water present in them, in other wordsshipping Water is not economical. On the other hand, a liquidcomposition can be dissolved in the bath in which it is to be used morequickly than a solid composition, and, if desired, may be initially madeup to a concentration suitable for immediate use; also, it contains in asingle mixture all of the ingredients, whereas in the case of a solidcomposition a single mixture constitutes a fire hazard due to thepresence of the acetateand CrO -containing source materials, and it ispreferred to make two packages to keep these materials separate.

Specific examples of the composition of matter are as follows:

Example 4 Parts by weight Sulfuric acid, H 50 Nitric acid, HNO Aceticacid, HC H O In carrying out the process, a zinc or zinc-surfacedarticle is dipped or immersed in an aqueous solution of the kinddescribed, kept immersed for about five seconds to two minutes, usuallyfrom 10 to 20 seconds, depending on the specific characteristics of thesolution as explained above, and then removed. The article is rinsed asby dipping in hot water. After the immersion treatment the article hasthe appearance of bright metallic zinc and is protected againstcorrosion, hot water, fingermarking, etc. by the transparent surfaceconversion coating.

The following examples illustrate the formation of coatings onzinc-surfaced articles, using various dip solutions.

Examples 7 to 14 A number of aqueous dip solutions were made up, theircompositions being set forth in the table below, wherein the numericalquantities are in terms of grams per liter. The symbol Ac representsacetate,

CH .COO

The solutions of Examples 7 to 11 were made by dissolving in waterchromic acid, sulfuric acid, sodium nitrate, and glacial acetic acid inamounts equivalent to the substances listed. Similarly, the solution ofExample 12 was made from chromic acid, sodium bisulfate, sodium nitrate,and acetic acid; that of Example 13 was made from chromic acid, sulfuricacid, nitric acid, and acetic acid; and that of Example 14 was made fromchromic acid, sulfuric acid, sodium bisulfate, sodium nitrate, aceticacid and sodium hydrogen acetate. The pH values shown were obtained byadjustment of the original solutions by additions of sodium hydroxide;in thecase of Example 13, no adjustment of the original solution wasmade.

Example 7 8 9 10 11 12 13 14 Pieces of zinc-coated steel of uniformsize, having a zinc coating thickness of about 0.0004: inch, were dippedin the solutions of Examples 7 to 11 for a period of 30 seconds, rinsedin water, then dipped in an aqueous solution containing 4 oz. per gallonof sodium hydroxide to help neutralize traces of adhering dip solution,and rinsed in warm water at a temperature of about 150" PL; in Example12, two pieces were dipped, one for 10 seconds and the other for 30seconds, and both were rinsed in hot water; and in Example 13 the dipwas for 10 seconds and the piece was rinsed in. boiling water. Dippingwas carried out at room temperature in. 400 cc. of each solution. Thecoatings produced in Examples 8, 9, 10, 11, and 13 were carefullyexamined. and found to be bright, transparent, protective coatings. Whenthe coatings were viewed at a right angle, ie., looked atperpendicularly, a faint. bluish color. was, apparent; the coatingswere. visible as iridescent coatings'when viewed at an acute angle.These coatings did not crack on contact with hot water and wereresistant to fingermarking. The pieces from Examples 7 and 12, which hadcoatings like the preceding pieces, wereimmediately subjected tostandard salt spray tests; after 16 hours the piece from Example 7 hadwhite corrosion products at its edge portions; the two pieces fromExample 12 resisted the salt spray for 4'6 hours, when small black spotswere apparent on them, and after 70 hours white corrosion products werepresent. As is known, uncoated zinc corrodes readily when exposed toatmosphere, so that in the foregoing salt spray tests, in which severeaccelerated conditions of corrosion obtain, the resistance of the coatedzinc for even one hour, or less, shows that the coating would protectthe zinc for substantial periods against usual atmospheric conditions.

The solution of Example 14, and also that of Example 12, was made up inquantity: 55 liters and 53 liters, respectively. Two zinc-coated wirerefrigerator shelves, 10 x 14", were dipped in the Example 14 solutionfor 10 seconds, rinsed in hot water, and found to have bright,transparent, protective coatings of the type described above. A shelfwas dipped in the solution of Example 12, being given a 10 second dip,water rinse, dipped in an aqueous solution containing g./l. of sodiumhydroxide, and rinsed in water. It had a coating of the type described.It was then subjected to a salt spray test, and after 16 hours it showedsome white products on the larger wires and some blotches on the smallerwires. Suflicient acetic acid was then added to the Example 12 solutionto raise the acetate concentration to 60 g./l., a shelf was dipped inthe resulting solution for seconds, rinsed in water, and found to have abright, clear protective coating.

This application is a continuation-in-part of application Serial No.641,864, filed January 17, 1946, and of application Serial No. 23,874,filed April 20, 1948, both now abandoned.

In the light of the foregoing description, the following is claimed:

1. A process of coating zinc surfaces which comprises immersing azinc-surfaced material in an aqueous solution of a plurality ofcompounds which provide the substances, and the amounts of saidsubstances, hereinafter described, said compounds being selected fromthe group consisting of salts and acids, the entire amount of one ofsaid substances being provided by at least one salt, said solutionconsisting essentially of the following said substances dissolvedtherein in the following amounts:

Grams per liter cro 48.1 to 288.5 504 5 l0 No 2 to 40 CH3.COO Up to 80said solution having a temperature from 60 to 130 F. and a pH between0.3 and +1.0, the immersion of the said material. being continued untila bright, colorless transparent protective coating; is imparted to thezinc surface, and. removing, the material from. the solution.

2. An. aqueous solution for coating zinc surfaces with a bright,colorless transparent protective coating, said solution having dissolvedtherein a plurality of compounds which. provide the. substances, and theamounts of said substances, hereinafter described, said compounds beingselected from the group consisting of salts and acids, the entire amountof one of said substances being provided by at least one salt, saidsolution consisting essentially of the following said substancesdissolved therein in the following amounts:

Grams per liter said solution having a temperature from 60 to 130 F. anda pH between -0.3 and +1.0.

3. A, mixture of" compounds for making up a solution for coating zincsurfaces with a bright, colorless, transparent, protective coating, saidcompounds providing said mixture with the substances, and the amounts ofsaid substances, hereinafter described, at least 50% byweight of saidsubstances being provided by at least one compound selected from thegroup consisting of acids and acid salts, the balance of said substancesbeing provided by at least one compound selected from the groupconsisting of acids, acid salts, and normal salts, the entire amount ofone of said substances being provided by at least one salt, said mixtureconsisting essentially of the following said substances in the followingamounts:

Parts by weight cro, 48.1 to 288.5 so 5 to 60 N0 2 to 40 cH,.co0 Up toGrams per liter CrO 48.1 to 288.5 so, 5 to 60 NO 2 to 40 CH .COO Up to80 the quantity of said mixture and of said water being correlated toprovide the foregoing amounts of said substances in said solution.

4. The mixture of claim 3 characterized by being normally solid.

5. The mixture of claim 3 characterized by being normally liquid.

6. A process of coating zinc surfaces which comprises immersing azinc-surfaced material in an aqueous solution of a plurality ofcompounds which provide the substances, and the amounts of saidsubstances, hereinafter described, said compounds being selected fromthe group consisting of salts and a mixture of salts and acids, saidsolution consisting essentially of the following said substancesdissolved therein in the following amounts:

Grams per liter the solution having a temperature from 60 to F. and a pHbetween -0.3 and +1.0, the immersion of the said material beingcontinued until a bright, colorless Grams per liter CrO 48.1 to 288.580,; to 60 N0 2 to 40 CH .COO Up to 80 the solution having a temperaturefrom 60 to 130 F.

and a pH between -0.3 and +1.0.

8. A mixture of compounds for making up a solution for coating zincsurfaces with a bright, colorless, transparent, protective coating, saidcompounds providing said mixture with the substances, and the amounts ofsaid substances, hereinafter described, said compounds being selectedfrom the group consisting of salts and a mixture of salts and acids,said mixture consisting essentially of the following substances in thefollowing amounts:

Parts by weight CrO 48.1 to 288.5 S0 5 to 60 N0 2 to CH .COO Up to 80said mixture being characterized by dissolving in water to produce asolution having a pH between -0.3 and +1.0 and consisting essentially ofthe following substances in the following amounts:

Grams per liter CrO 48.1 to 288.5 so, 5 to NO 2 to 40 cn .coo Up to sothe quantity of said mixture and of said water being correlated toprovide the foregoing amounts of said substances in said solution.

References Cited in the file of this patent UNITED STATES PATENTS2,497,905 Ostrander Feb. 21, 1950 Ostrander Feb. 17, 1953

1. A PROCESS OF COATING ZINC SURFACES WHICH COMPRISES IMMERSING AZINC-SURFACED MATERIAL IN AN AQUEOUS SOLUTION OF A PLURALITY OFCOMPOUNDS WHICH PROVIDE THE SUBSTANCES, AND THE AMOUNTS OF SAIDSUBSTANCES, HEREINAFTER DESCRIBED, SAID COMPOUNDS BEING SELECTED FROMTHE GROUP CONSISTING OF SALTS AND ACIDS, THE ENTIRE AMOUNT OF ONE OFSAID SUBSTANCES BEING PROVIDED BY AT LEAST ONE SALT, SAID SOLUTIONCONSISTING ESSENTIALLY OF THE FOLLOWING SAID SUBSTANCES DISSOLVEDTHEREIN IN THE FOLLOWING AMOUNTS: